John Key said the toll was expected to rise further, adding: “We may be witnessing New Zealand’s darkest day.”
The tremor caused widespread damage as it occurred at a shallow depth of 5km (3.1 miles) during lunchtime when Christchurch was at its busiest.
The mayor of New Zealand’s second-biggest city says 120 people have been rescued from the ruins.
Screams from rubble
The damage is said to be far worse than after the 7.1-magnitude quake on 4 September, which left two people seriously injured but no fatalities.
We’ve had friends in town call us and say there are just bodies lying around: lots of dead bodies outside shops just lying there just covered in bricks”
The epicentre of that quake, which occurred in the middle of the night, was further away from the city and deeper underground, but it still caused an estimated $3bn (£1.9bn) in damage.
Shocked survivors could be seen wandering the rubble-strewn streets, which cracked open as the ground beneath was liquefied by the tremor.
Police said that the dead included people on two buses which were crushed by falling buildings.
Helicopters plucked survivors to safety from rooftops, and dumped water on fires.
Officials said up to 30 people were feared still trapped inside the razed Pyne Gould Guinness building, where screams have been heard from the ruins.